1. Field of the Invention
The invention relates to techniques for obtaining bent and tempered motor vehicle glazings, according to which the glass sheets are heated to the bending temperature in a horizontal tunnel furnace, are bent to a desired shape, in a horizontal position, by pressing the glass sheets between a solid upper form and a lower form of the frame type, the lower form having several parts that are mobile relative to one another, at least one of the parts being bent during pressing, then carrying the bent glass sheets on a conveying and tempering frame corresponding to the shape of the bent glass sheets into a tempering station where they are tempered by blowing cold air.
2. Description of the Related Art
A process of this type is known from U.S. Pat. No. 4,661,141. The use of bending frames in several parts has proven necessary for the production of glazings of complex shapes. According to the above-mentioned document, after the bending, the glass sheet is kept against the upper form by suction. The bending frame is lowered and brought by a side outlet outside the bending station. The glass sheet is then taken over by a conveying frame in a single part whose profile corresponds to the bending profile imparted to the glass sheet, then the bent glass sheet is carried on this conveying frame into the tempering station. During the operation of tempering by blowing cold air, the glass sheet is supported all along its periphery by this frame which consists of a single nondeformable part.
Under certain conditions, this known process can be used according to a simplified mode; according to the latter, the lower bending form made up of several parts also serves as a conveying and tempering frame. This makes it possible to shorten the cycle time and to simplify the device overall.
This variant embodiment, in which one and the same frame is used for bending and tempering, unfortunately cannot be used for the production of glazings exhibiting a pronounced bending close to their edges. In this case, deformations appear at the very bent marginal zones, these deformations are in the form of small undulations that create optical defects. Analysis shows that these undulating deformations are due both to the bending tool and to the mode of cooling in the tempering station and that they cannot be eliminated by a single adjustment of the parameters of the process within the context of usual well-known adjustment measures.
Also, from the patent DE-A-35 25909 a process of bending and tempering motor vehicle glazings is known according to which the glass sheets, heated in a horizontal tunnel furnace, are brought in a horizontal position between an upper form and a lower form of several parts and, after the bending process by pressing, are directly tempered in the bending station by cold air blown in the direction of the glass sheets through orifices made in the bending tools. The lower bending form consists of an outside frame in several parts, connected together by joints, surrounding an inside solid form provided with orifices for the blown air. The contact surface of the bending form, i.e., the surface of the lower form surrounded by the outside frame, against which the glass sheet is actually flattened, is covered with a mat of refractory fibers. During the tempering process, the glass sheet is supported only by the outside frame. Further, this process does not apply to the production of glass sheets comprising very bent marginal zones.
Finally, it is known from patent application DE-A-35 41773 to cover solid bending forms with a felt mat preferably exhibiting a thickness between 3 and 10 mm and which contains, among others, thin steel fibers.